Fuel-injected internal combustion engines are used in many applications including the diesel-electric drive systems of railroad locomotives. FIG. 1 illustrates a load control system 10 for the electromotive drive system of a locomotive as provided by the assignee of the present invention. The engine is operated at a constant speed that is dependent upon a power demand (commonly referred to as the throttle notch setting) initiated by the locomotive operator. The speed is regulated to the speed command value 12 by adjusting the amount of fuel delivered to the engine. A speed regulator 14 generates a fuel demand signal 16 based upon the speed command 12 and an actual engine speed feedback signal 18. During normal operation, the fuel demand 16 is converted directly to fuel flow 20. However, under some conditions the amount of fuel 20 must be limited from that associated with the fuel demand 16 in order to prevent overloading of the engine and to accommodate failures of engine components and associated equipment. A fuel limit function 22 depends upon two different criteria. One is a static limit 24 based upon the current engine speed, temperature and pressure at various locations. The static limit 24 protects the engine and associated systems from mechanical overloading. A second criterion is a dynamic limit 26 required to meet transient limits such as smoke or other emissions or to account for the turbocharger lag. The lower of the static limit 24 and dynamic limit 26 is selected by a minimum function 28 as an input to the limit function 22 to limit the amount of fuel flow 20 when the fuel demand is higher. When the limit function 22 is active, the engine will receive less fuel than required to maintain the demand speed command 12 and the actual speed 18 will drop unless further control action is initiated. In order to prevent such a drop in actual engine speed 18, a load control function 30 senses the difference between the fuel flow 20 and the fuel demand 16 and provides a load reduction signal 32 to a minimum function 34 to be compared to operator demand signal 36. The operator demand signal is associated with the throttle notch setting and speed command 12. The minimum function 34 provides an output to the traction motor load control 38 to produce a load control signal 40 to control the alternator used to power the main locomotive traction motors. The reduced load imposed on the engine by the alternator counteracts the reduction in fuel flow 20 caused by the limit function 22, thereby allowing the engine actual speed 18 to be maintained consistent with the speed command 12, albeit at a lower than normal power output level.
The amount of fuel required to produce full horsepower does not remain constant over the life of an engine. Short-term variables such as ambient temperature, ambient pressure and fuel type/quality affect the amount of fuel used. Over the longer term, component wear will reduce the efficiency of an engine and will result in an increase in the amount of fuel used. Maintenance activities such as the replacement of parts will also change the amount of fuel used. The static limits 24 and dynamic limits 26 must be set sufficiently high to accommodate such short and long-term changes. In present locomotive engine designs, these limits may be set 50% above the initial fuel consumption level, to account for these changes.
A typical locomotive engine may have 12 or 16 cylinders. When one of the cylinders and/or the associated fuel delivery path fails, the prior art load control system 10 of FIG. 1 will increase the total fuel flow in order to maintain the desired engine speed on the remaining 11 or 15 cylinders. The static limit 24 ensures that the increased fuel flow is not so high as to cause immediate catastrophic failure of the engine. However, the increased fuel delivery to the remaining working cylinders will cause higher than normal stress, thereby causing higher overall failure rates on the affected components. Exhaust emissions may also be affected. Such failures are most likely to occur if the failure remains undetected by the operator and the engine is operated in this degraded mode for an extended period.